May has called for rules on MPs' behaviour to be toughened after the emergence of several allegations of sexual harassment at Westminster.
He became swept up in the Westminster sexual harassment scandal when it emerged he had felt the knee of presenter Julia Hartley-Brewer at a dinner in 2002.
The shock resignation of Sir Michael Fallon after allegations of improper behaviour opened up a big role in cabinet, and Williamson wasn't one of the names floated as a potential replacement.
Scottish Tory leader Ruth Davidson said the "tone of politics had changed".
He is the first politician to quit following recent claims of serious sexual abuse in United Kingdom parliament.
In his resignation letter to U.K. Prime Minister Theresa May, Fallon said, "A number of allegations have surfaced about MPs in recent days, including some about my previous conduct".
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He told the BBC that what had been "acceptable 15, 10 years ago is clearly not acceptable now".
"I have therefore reflected on my position in Government and I am resigning as Defence Secretary".
Sir Michael resigned from the Cabinet on Wednesday evening following allegations he had behaved inappropriately. The Telegraph understands that there will not be a reshuffle but that Sir Michael will be directly replaced.
Labour, meanwhile, has launched an independent investigation into an activist's claim that she was discouraged by a party official from reporting an alleged rape at a party event in 2011.
The prime minister, who has long championed the careers of female lawmakers, said on Wednesday that action would be taken when there were allegations and evidence of sexual misconduct.
"We have a duty to ensure that everyone coming here to contribute to public life is treated with respect."